Expansion of fluid

When a liquid is heated it expands, i.e. its density falls as temperature increases. The temperature of a liquid in a heating system will vary significantly over time.

When a liquid is heated it expands, i.e. its density falls as temperature increases. The temperature of a liquid in a heating system will vary significantly over time. Typically, this can be between 20–80 °C, which will cause an increase in volume of approx. 3 %. Each heating system must be designed to accommodate its specific expansion requirements.

There are two types of heating system expansion systems:

Open expansion

Closed expansion

Open expansion
Open expansion is a system where each boiler is connected to the outside atmosphere by a safety conduit. The safety conduit must never be closed.
The safety conduit can be directed over the roof, either via an expansion tank or directly.

Closed expansion
A closed system is a system where expansion is absorbed in a pressure expansion tank. The expansion tank is usually fitted with a diaphragm that separates the liquid from the air in the tank. The closed expansion system must also be fitted with a relief valve.

Solid-fuel systems are typically fitted with open expansion on the primary side and a closed expansion on the secondary side.